1. Field of the Invention
The present invention relates generally to a knock detecting system for internal combustion engines. More specifically the invention relates to a knock detecting system which can determine knock intensity on the basis of heat quantities produced in the combustion chamber of the engine.
2. Description of the Background Art
Since electronic control techniques have been widely used for vehicle and engine controls, a variety of methods for decreasing and controlling knocking in internal combustion engines have been used. In conventional methods, knocking is decreased by improving the shapes of combustion chambers, gas flows in the combustion chambers or the like, or by increasing the octane number of the fuel used. In recent years, in addition to the aforementioned conventional methods, a new knock control technique, which can be used in unmodified stock cars, has been proposed. In the proposed technique, knocking control is performed by causing the spark ignition timing to approach the limit at which slight knocking, undetectable to human ears, occurs, so that improved fuel consumption and power performance can be obtained under all driving conditions, regardless of a difference between fuel characteristics and/or a change of octane number required for the engine with the passage of time.
In recent years, a system has been proposed in which this new knock detecting technique is used on respective cylinders of a 6-cylinder internal combustion engine. In addition, in on-board knock control systems using electronic control, knock detection and quantitative processing techniques are important.
Methods for detecting knocking, which can be used for automotive vehicles, are classified by detected physical quantities. In such methods, there are various methods which utilize pressure in engine cylinders vibration of the engine, combustion light produced in the combustion chamber, noise level, ion current in engine cylinders, and so forth.
Among the aforementioned methods, one typical conventional knock detecting system for internal combustion engines is described in, for example, "Automobile Technique, 1986, Vol. 40, No. 11". This system detects high-frequency components near a knock frequency by fluctuation in a pressure waveform in the combustion chamber on the basis of output from a cylinder pressure sensor mounted on an ignition plug, assigning numerical values to the intensities of the high-frequency components of the fluctuation waveform for determining knock level using a statistical process.
However, in this and other conventional knock detecting systems for internal combustion engines, a logic used for knock detection must be adjusted for respective types of engines, since the fluctuation detected by the cylinder pressure sensor is influenced by mounting position, the type of sensor, and which cylinder the detection occurs in. As a result, there is a disadvantage in that man-hours and costs are increased. In addition, when the adjustment of logic for knock detection does not exactly meet engine requirements, there is a disadvantage in that the accuracy of knock detection is decreased.
To use cylinder pressure sensors for conventional knock detecting methods is very effective since the engine requires no special processing. However, in case a where a knock detecting method uses high-frequency components of signals output from a sensor, it is difficult to separate a noise component from the output signals, since the output signals do not include only combustion pressure, but also vibration of the engine itself and mechanical vibration caused by rotation. Therefore, there is a disadvantage in that normal knock components can not be accurately detected. Thus, sufficient accuracy in knock detection can not be obtained. For example, when engine speed becomes high, fluctuation in the high-frequency components of the cylinder pressure increase, in such case it becomes particularly difficult to separate knock components from other signal components.